It has long been recognized as a difficult and expensive proposition to produce molds for products having complex geometric configurations. The problems are compounded when the configurations must be formed on surfaces which are of complex geometry or constitute combinations of complex geometric areas. Molds for the shaping and curing of pneumatic tires are a classic example in that they are of a generally toroidal configuration having a pair of interior surfaces defining the sidewalls of a tire, which are spaced and joined by a tread portion of the mold. While a tire tread is circular in a direction circumferentially thereof, the tire tread normally has a plurality of areas of different curvature in a direction laterally or axially of the tire tread.
Tire treads have multiple projections which form what are commonly referred to as grooves and sipes that are normally in circumferential, repeating patterns about the entire periphery of the tread. The arrangement of the grooves and sipes is material to the traction, water removal, noise and other characteristics of a tire. There are an infinite number of possible arrangements of grooves and sipes on a tire which may be employed to achieve desired operating characteristics for particular tire designs. Many thousands of tire tread configurations have been employed by tire manufactures over the years. While the arrangement, depth, and size of grooves and sipes in tire treads are subject to infinite variation, they do share the common trait that the sides thereof are disposed substantially normal to the curved surface of the tire tread at their location thereon so that the grooves and sipes are substantially perpendicular to the ground surface when that portion of the tire tread comes in contact with the ground.
In order to achieve this final configuration of the completed tire, the tire molds have projecting ribs that form the grooves in the tread and sipe blades which form the sipes in the tread. In order to achieve the desired orientation of the grooves and sipes, the ribs and sipe blades must be perpendicular to the tread contour surface on the mold which forms the outer tread surface of the tire. Since the contour surface of a mold is of complex curvature in that there is a uniform curvature circumferentially of the tire and variable curvature laterally thereof, it becomes a complex three-dimensional, geometric problem to construct a tire mold having the sides of the ribs and sipe blades perpendicular to the tread contour surface at substantially all points thereon. In some instances, it is intentionally desired that the side of the ribs or sipe blades be at an angle to the contour surface to achieve what is termed a draft angle, which further complicates the already complex mold forming processes that are employed in the art.
In order to achieve even an acceptable degree of precision in tire molds, it has historically been necessary to employ highly complex molding processes. At various times, tire molds have been made employing plaster molding, sand casting, ceramic casting, metal mold casting, and other precision casting processes. Due to the number of steps and inherent irregularities and variations, the aluminum molds normally produced in this manner lack precision and commonly exhibit defects despite the very high manufacturing costs attendant such multi-step molding processes. In other instances, engraving processes have been employed to make tire molds of materials which are harder and more durable than aluminum, such as steel. In many instances, it has nonetheless been desirable or necessary to provide detachable or permanently attached inserts to form the ribs and sipe blades which project inwardly from the tread contour surface of the mold. The use of such inserts, however, creates the possibility of tire tread rubber being forced between the tread contour surface and the insert during shaping and curing of uncured tires to produce a burr or defect in the finished tire. In other instances, it has become common to make sipe blades as a separate element and insert them in grooves formed in the tread contour surface. This, however, requires an additional labor-intensive step and produces a mold wherein the sipe blades may be loosened or dislodged after a period of usage, thus creating significant mold servicing or rebuilding expenses and a loss of operating time for the mold.
More recently, substantial interest has developed in the making of tire molds employing electrical discharge machining techniques because the molds may be made of hardened steel, with the attendant advantages of durability, longevity, and the limited need for servicing or rebuilding. In some instances, efforts have been made to electric discharge machine inserts which are positioned in a tread contour surface. However, this approach is subject to the longstanding problem that such inserts may be damaged, become loosened, create defects as described above, or move sufficiently such that the accuracy of the mold is affected.
In order to avoid these disadvantages, efforts have been made to electric discharge machine tire molds from integral parent material such as to avoid the disadvantages of inserts of any type. These efforts have encountered the problems that in endeavoring to form the ribs and sipe blades having the normally intricate patterns, it is necessary that multiple burning operations be undertaken with different approach angles, and in some instances different tooling, in order to achieve sufficient precision of the sides of the sipe blades and ribs without the incidental removal of material constituting required portions of the sipe blade or rib elements. The necessity of multiple electric discharge machining stages and/or a plurality of burns per stage, often with different electrode configurations, has resulted in processes which are of sufficient duration and require a sufficient number of different electrode configurations such as to make the process excessively expensive and frequently applicable to only a limited number of geometric configurations of the sipe blades and ribs.
Thus, none of the known processes for the making of tire molds have solved the various problems attendant the various processes as described above at less than an extremely high cost while achieving accuracy which remains below tire industry goals. The criticality of these problems has been intensified by the fact that tire manufactures have come to realize the importance of highly accurate mold dimensions because such accuracy is highly significant in terms of improved performance and wear achieved by the resultant tires.